“GBB” type, flat hopup rubber, 4 hardness set (50°/60°/70°/80°) Initially designed for the TAC-41 “GBB” hopup, this hopup rubber can fit inside most airsoft guns using “GBB” hopup rubber. Transparent silicone for easier installation, this kit includes 4 hardness: – blue color, 50°– green color, 60°– yellow color, 70°– red color, 80°arge)*